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    Subject[PATCH 6/7][v8] Protect cinit from blocked fatal signals

    From: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    Date: Wed, 24 Dec 2008 14:04:24 -0800
    Subject: [PATCH 6/7][v8] Protect cinit from blocked fatal signals

    Normally SIG_DFL signals to global and container-init are dropped early.
    But if a signal is blocked when it is posted, we cannot drop the signal
    since the receiver may install a handler before unblocking the signal.
    Once this signal is queued however, the receiver container-init has
    no way of knowing if the signal was sent from an ancestor or descendant
    namespace. This patch ensures that contianer-init drops all SIG_DFL
    signals in get_signal_to_deliver() except SIGKILL/SIGSTOP.

    If SIGSTOP/SIGKILL originate from a descendant of container-init they
    are never queued (i.e dropped in sig_ignored() in an earler patch).

    If SIGSTOP/SIGKILL originate from parent namespace, the signal is queued
    and container-init processes the signal.

    IOW, if get_signal_to_deliver() sees a sig_kernel_only() signal for global
    or container-init, the signal must have been generated internally or must
    have come from an ancestor ns and we process the signal.

    Further, the signal_group_exit() check was needed to cover the case of
    a multi-threaded init sending SIGKILL to other threads when doing an
    exit() or exec(). But since the new sig_kernel_only() check covers the
    SIGKILL, the signal_group_exit() check is no longer needed and can be
    removed.

    Finally, now that we have all pieces in place, set SIGNAL_UNKILLABLE for
    container-inits.

    Changelog[v6]:
    - Add a note regarding the signal_group_exit() in patch description.

    Changelog[v5]:
    - (Oleg Nesterov) Drop signal_unkillable(), simplify check in
    get_signal_to_deliver() and drop check for signal_group_exit()
    since it is covered by sig_kernel_only().

    Changelog[v4]:
    - Rename sig_unkillable() to unkillable_by_sig()
    - Remove SIGNAL_UNKILLABLE_FROM_NS flag and simplify (Oleg Nesterov)
    - Set SIGNAL_UNKILLABLE for container-init in this patch.

    Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
    ---
    kernel/fork.c | 2 ++
    kernel/signal.c | 9 ++++++++-
    2 files changed, 10 insertions(+), 1 deletions(-)

    diff --git a/kernel/fork.c b/kernel/fork.c
    index 2b45bc5..ddfa413 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -841,6 +841,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
    atomic_set(&sig->live, 1);
    init_waitqueue_head(&sig->wait_chldexit);
    sig->flags = 0;
    + if (clone_flags & CLONE_NEWPID)
    + sig->flags |= SIGNAL_UNKILLABLE;
    sig->group_exit_code = 0;
    sig->group_exit_task = NULL;
    sig->group_stop_count = 0;
    diff --git a/kernel/signal.c b/kernel/signal.c
    index a3a1304..c94355b 100644
    --- a/kernel/signal.c
    +++ b/kernel/signal.c
    @@ -1862,9 +1862,16 @@ relock:

    /*
    * Global init gets no signals it doesn't want.
    + * Container-init gets no signals it doesn't want from same
    + * container.
    + *
    + * Note that if global/container-init sees a sig_kernel_only()
    + * signal here, the signal must have been generated internally
    + * or must have come from an ancestor namespace. In either
    + * case, the signal cannot be dropped.
    */
    if (unlikely(signal->flags & SIGNAL_UNKILLABLE) &&
    - !signal_group_exit(signal))
    + !sig_kernel_only(signr))
    continue;

    if (sig_kernel_stop(signr)) {
    --
    1.5.2.5


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